Magnetometer for measuring the magnetic moment of a specimen

ABSTRACT

In a magnetometer with an inhomogeneous magnetic field which timewise changes in its intensity periodically and acts upon the specimen to be measured, the latter being attached to the free end of a tongue (8) that can be oscillated and whose oscillations can be determined by piezo elements (12, 13), there occurs a compensation of the magnetic moment of the specimen with the aid of a compensating current loop (33). The current required for arresting the oscillations is a measure of the magnetic moment to be determined. An adaptation to the resonance frequency of the oscillating tongue (8) is effected through a frequency control cirucit (22, 28, 6), making it possible to measure large temperature ranges at great sensitivity and contingent on temperature.

The invention concerns a magnetometer with an inhomogeneous magneticfield which timewise changes in its intensity periodically and acts onthe sample to be measured, which sample is attached to the free end of atongue that can be caused to oscillate and whose oscillations can berecorded by at least one piezo element connected with an evaluationcircuit that features a measuring amplifier for acquisition of theamplitudes of the piezo signals, and features a low frequency generatorwhich through a power amplifier is connected to a coil setup whichgenerates the periodically changing magnetic field, with the samplebeing surrounded by a compensating current loop.

A magnetometer of that type, without compensating current loop, isdescribed in "Review of Scientific Instruments" 51 (5), May 1980, pages612, 613 and enables qualitative measurements of the magnetic moments ofvery small samples, by registering with the aid of a piezo element theoscillation amplitude of a tongue constructed from gold wire and a glassfiber. The output signal of the piezo element is fed to aphase-sensitive amplifier whose output signal represents the measuringsignal.

The oscillation amplitude of the prior system, and thus the outputsignal, depend not only on the magnitude of the magnetic moment to bedetermined but also on the elasticity of the materials used the inertiaof the oscillator and the magnitude as well as frequency of the fieldgradient Therefore, only qualitative measurements for microscopicallysmall samples, without registering the temperature contingency of themagnetic moment, are possible with the prior magnetometer.

From the 1984 activities report of the Fraunhofer Institute for appliedsolid state physics, page 136, it is known that the piezo signals can becompensated for by the magnetic moment of a current loop. How this is tobe accomplished in practice, however, does not derive from theactivities report.

Basing on this prior art, the problem underlying the invention is toprovide a magnetometer of the initially mentioned type which allows theperformance of quantitative measurements of magnetic moments at varioustemperatures, also on samples having dimensions in the range of severalmillimeters.

This problem is inventionally solved in that the low frequency generatoris activated by an automatic frequency control circuit by which thefrequency of the magnetic field can be adjusted to the respectiveresonance frequency of the tongue, and in that the magnitude of thecompensating direct current can be registered by the compensationcurrent loop for the quantitative determination of the magnetic moment,at which compensating current the magnetic moment of the sample, andthus the excitation of the tongue, is compensated for.

Since a measurement takes place after the tongue oscillations have beenzeroed, a quantitative determination of the magnetic moment of a sampleis possible independently of the amplitude of the output signal of themeasuring amplifier and thus independently of the system magnitudeswhich have an influence on the amplitude. By tuning the excitingfrequency to the resonance frequency it is possible to measure magneticmoments across large temperature ranges at a very high sensitivity.

In a favorable embodiment of the invention, the sample is contained in ahollow cylinder from quartz whose longitudinal axis extends in thedirection of the magnetic field between the two magnetic poles and onwhich a wire loop with a winding from a thin gold wire is applied ascompensating current loop. The tongue to be oscillated with the hollowcylinder from quartz is housed in a tail part of a cryostat whichprotrudes into the gap between the two poles of the electromagnetgenerating the magnetic field.

The measuring amplifier is preferably a phase-sensitive amplifier with afirst output for a first control circuit that contains the compensatingcurrent loop. A second output of the phase-sensitive amplifier forsignals phase-shifted by 90° controls the input of a circuit for anautomatic frequency control which is located in a second control circuitwhich makes it possible to tune the excitation frequency of the lowfrequency generator to the respective resonance frequency of theoscillation system.

Suitable developments and advancements of the invention are set forth inthe subclaims.

An embodiment of the invention will be more fully explained hereafterwith the aid of the drawings.

FIG. 1 shows schematically the inventional magnetometer together with ablock diagram of the control circuits;

FIG. 2, the cryostat of the magnetometer in side elevation, partly cutaway;

FIG. 3, a longitudinal section of the cryostat tail along line III--IIIin FIG. 4;

FIG. 4, a cross section of the cryostat tail along line IV--IV in FIG.3;

FIG. 5, the tongue of the magnetic sensor in a partly cut-away sideelevation, and

FIG. 6, a view of the tongue of the magnetic sensor of the magnetometerin the oscillation direction of the tongue and quartz hollow cylinder ofthe magnetometer.

Illustrated schematically in FIG. 1 along with a block diagram of theelectronic components, the magnetometer features an electromagnet withpole shoes 1, 2 on which gradient coils 3, 4 are arranged which inexcited condition generate between the pole shoes 1, 2 a magnetic fieldgradient which timewise varies in its intensity, sinusoidally, accordingto the current supplied to the gradient coils 3, 4 by a low frequencyamplifier 5. The low frequency amplifier 5 is connected with the outputof a low frequency generator 6. The excitation frequency that can begenerated by the low frequency generator 6 ranges between 10 and 100 Hz,so that the timewise varying magnetic field between the gradient coils 3and 4 will oscillate at the same frequency.

Located halfway between the pole shoes 1, 2 is a hollow cylinder 7 fromquartz serving as space for the specimen whose magnetic moment is to bedetermined quantitatively. Due to the magnetic field gradient, thesample mounted in the quartz hollow cylinder 7 is subjected to a forcewhose magnitude depends on the magnetic moment and the field gradientDue to the alternating current fed to the gradient coils 3, 4, a forcewhich periodically changes in intensity and direction acts thus on thesample along the connecting line between the pole shoes 1 and 2.

The hollow cylinder 7 from quartz is fastened to the front, free end ofthe tongue 8 which due to the magnetic excitation is caused to oscillatewhile fixed on the end opposite the quartz hollow cylinder 7, by a rigidholding tube 9 from ceramic.

The tongue 8 consists of two quartz tubelets 10, 11 which are connectedwith the quartz hollow cylinder 7 and connected on the end opposite thequartz hollow cylinder 7 with reglets 12, 13 which consist ofpiezoceramic material and serve as piezo elements for registering theoscillating movements of the tongue 8.

The reglets 12, 13 that are effective as piezo elements are connectedthrough lines 14, 15 with the input of charge/voltage converters 16, 17in such a way that signals of opposite phase will be carried by theoutput lines 18, 19 as the tongue 8 oscillates.

The opposite-phase signals energize the two inputs of a subtractingstage 20, so that the opposite-phase signals will add and produce on theoutput 21 an output signal which is stronger as compared to the inputsignals, with equal-phase interference signals contained in the outputlines 18, 19 being suppressed.

The output 21 is connected with the input of a phase-sensitive amplifier22 (lock-in amplifier) which through a reference input 23 is connectedwith the low frequency generator 6 in order to make available to thephase-sensitive amplifier 22 the necessary reference frequency, which isidentical with the excitation frequency of the tongue 8.

The phase-sensitive amplifier 22 features a first output 24 for acompensating current loop and a second output 25 input for signalsphase-shifted by 90° . As the tongue 8 oscillates, the output 24 carriesa direct voltage signal whose magnitude depends on the amplitude of theoscillations of the tongue 8 which, in turn, depends on the magneticmoment of the sample.

The first output 24 of the phase-sensitive amplifier 22 connects througha first switch 26 with a proportional/integral member or proportionalintegrator 29. The second output 25 connects through a second switch 27with an automatic frequency control circuit 28 that can be varied by wayof the excitation frequency of the low frequency oscillator 6.

When the second switch 27 is closed while the first switch 26 is opened,a closed control circuit results that tunes the excitation frequency tothe resonance frequency of the magnetic sensor which includes the tongue8. Depending on the specimen mass and the temperature, e.g., thereference frequency ranges between 40 and 80 Hz. The occurrence of aresonance causes a maximum deflection of the tongue 8, and the amplitudereaches on the first output 24 a maximum. The signal on the secondoutput 25, contrarily, is zero at balanced phase position. When a changeof the resonance condition of the magnetic sensor with the tongue 8occurs, for instance due to temperature change, a change of the phaserelationship between the excitation and the oscillation of the tonguewill result, expressing itself as a signal on the second output 25 whichdiffers from zero and is positive or negative, depending on thedirection of variation. With the second switch 27 closed, this signalproceeds to the automatic frequency control circuit 28 which continuesto tune in on the excitation frequency of the low frequency oscillator6, in the direction given by the respective sign of the signal, untilthe original phase relationship and thus the resonance between theexcitation frequency and the natural frequency of the magnetic sensor isrestored.

By tuning the excitation frequency to the resonant frequency andalternately actuating the first switch 26 for measuring the magneticmoment in the manner described hereafter, and actuating the secondswitch 27 for tuning to the resonance conditions of the magnetic sensorit is possible to measure magnetic moments of the sample at very highsensitivity and contingent on temperature, across large temperatureranges, for instance from 5 to 350 K.

The proportional/integral member 29 connected with the first switch 26energizes a current actuator or current source whose output is afunction of the input voltage, 30 whose current can be measured by acurrent measuring device 31 and which through a line 32 energizes a wireloop 33 with a compensating current, which loop is wound about theoutside of the quartz hollow cylinder 7. The magnetic moment generatedby the wire loop 33 causes at the selected winding and current directiona compensation of the alternating force acting on the tongue 8 of themagnetic sensor, due to the compensation of the magnetic moment of thespecimen. The oscillation amplitude of the quartz hollow cylinder 7decreases as the compensation increases, and at complete compensationthere occurs no longer any deflection of the tongue 8 and the directvoltage signal on the output 24 will be zero. The mutual dependency ofthe signal on the output 24 and the compensating current through thewire loop 33 defines a closed control circuit which through zerodetection of the signal on the output 24 and of an electric compensatingcurrent through the wire loop 33 serves to compensate for the forces onthe magnetic sensor with the tongue 8 and to tune the deflection tozero.

Upon zeroing of the signal on the output 24, in the off-condition of thecontrol circuit, the magnetic moment of a sample in the sample space canthus be determined solely by measurement of the electric current throughthe wire loop 33 around the sample space and the geometric dimensions ofthe loop. To that end, the magnetic moment of the empty specimen holder,as far as it interacts with the field gradient, needs to be determinedby a blank measurement and subtracted from the sum moment.

The measuring setup described above allows the quantitativedetermination of the magnetic moment of a specimen irrespective ofvariables which influence the amplitude, since the measurement takesplace through zero adjustment.

Coordinated with the current actuator 30 is a generator 34 that can beconnected with the current actuator 30 through a third switch 35 inorder to perform the mentioned compensating operation alternatively andmanually by an operator action without making use of the normally closedcontrol circuit comprising the phase-sensitive amplifier 22 and theproportional/integral member 29, by supply of an electric voltage, byclosing the third switch 35 while the first switch 26 is open, or,alternatively to generate another magnetic moment.

The generator 34 serves in an alternative way to perform thecompensation of the magnetic moment of the specimen by the magneticmoment of wire loop 33. Depending on the adjustment of the output ofgenerator 34 a compensation or an additional magnetic moment can beachieved. The value of the additional magnetic moment is determined bymaking a calculation which uses the current, the number of turns and thediameter of the wire loop 33.

When switch 27 is closed the closed circuit for adjusting the resonancefrequency is working. When switch 26 is open the closed circuit tocompensate the mechanical movement of the specimen is not workingautomatically, but it is possible to make a compensation by closingswitch 35 and by adjusting the output of generator 34 manually in such away that the current through line 32 and the wire loop 33 generates amagnetic moment which compensates the magnetic moment of the specimen.By taking into account the current measured by current measuring device31 it is easy to calculate the magnetic moment generated by the wireloop 33 using the current and the diameter of the wire loop and thenumber of turns of that coil.

The actuation of the switches 26, 27, 35 and the reading of the currentvalue from the current measuring device 31 can be effected through acomputer not illustrated in the drawing, enabling an automatic operationof the magnetometer.

The magnetic sensor illustrated in FIG. 1 in the area of the pole shoes1, 2 is located in the tail part 36 of a cryostat 37 that can be seen inFIG. 2 and may be fashioned as a helium flow cryostat. From FIG. 2 it isevident how the tail part 36 protrudes into the gap between the poleshoes 1, 2 with the gradient coils 3, 4. The cryostat 37 features acryostat external tube 38 which is firmly connected with a concreteplate 39 resting on air cushions not illustrated in the drawing. Visibleabove the concrete plate 39, in FIG. 2, is a coolant inlet socket 40 anda coolant outlet socket 41 which through coolant lines 42, 43 areconnected with a heat exchanger 44 which is located above the tail part36, in the lower section of the cryostat external tube 38.

Contained on the upper end of the cryostat external tube 38 is a springbushing 45 with a dual-stage spring system, symbolically illustrated bysprings 46, 47, for low-vibration mounting of a heavy mass 48 to whichthe holding tube 9 described in conjunction with FIG. 1 is attached,which supports the magnetic sensor arranged in the tail part 36.

FIG. 3 depicts an enlarged cross section of the tail part 36 of thecryostat 37, for illustration of further detail. Previously mentionedcomponents are marked using the same reference symbols as in FIGS. 1 and2.

As can be seen from FIG. 3, the tail part 36 of the cryostat 37 consistsof an external tube 49 which is connected with the bottom end of thecryostat external tube 38 by way of flange parts 50. Contained insidethe external tube 49 from fiber-reinforced plastic is a thermalshielding tube 51 which by its construction prevents the occurrence ofeddy currents that would be created by the gradient coils 3, 4 in ametal tube and would cause vibrations in the cryostat and affect themagnetic sensor. The thermal shielding tube 51 consists of a thin-walledtube 52, for instance from plexiglas which together with the tubes ofthe tail part 36 is illustrated, in FIG. 4, in cross section andsurrounded by a layer of densely packed enameled copper wires 53 with adiameter of, e.g., 0.55 mm which at a slight helix extend along thelongitudinal axis of the tube 52 in the way illustrated in FIG. 3, as asection. The layer of copper wires 53 is lapped with a thread 54 thatextends approximately at a right angle to the copper wires 53.

The bottom ends of the copper wires 53 are radially bent inward by 90°and clamped between two screw-tightened teflon disks 55, 56. The upperends of the copper wires 53 are tightened between a clamping ring 57 anda flange ring 58 by way of which the shielding tube 51 is mounted on thecoordinated flange 59 of the cryostat 37. This arrangement, for one,assures the necessary thermal conductivity along the entire length ofthe shielding tube 51 and, for another,prevents electrical current pathsthough with diameters that are greater than the diameter of the mutuallyinsulated wires.

Contained inside the shielding tube 51, as can be seen from FIGS. 3 and4, is a cryostat internal tube 60 that consists as well from glassfiber-reinforced plastic. The cryostat internal tube 60 is filled withhelium gas pressurized to about 10 mbars. Extending inside the cryostatinternal tube 60 are two copper troughs 61, 62 which serve as a coolingtube and are thermally well connected with the heat exchanger 44 of thecryostat. The shape of the copper troughs 61, 62 can be seen clearlyfrom the cross section illustrated in FIG. 4. The alignment of thelongitudinal gap formed between the copper troughs 61, 62 is parallel tothe drawing plane in FIG. 3, and thus along the field lines between thepole shoes 1, 2 of the electromagnet.

FIGS. 5 and 6, greatly enlarged, depict the magnetic sensor arrangedinside the cryostat internal tube 60, which is illustrated in FIGS. 1and 3.

Visible in FIG. 5 is the bottom end of the holding rod 9, which isfashioned as a flattened ceramic transition piece 63. Reglets 12, 13 areglued with their upper ends on the flattened ceramic transition piece63. Additionally, a thermal sensor tubelet 64 from quartz, foraccommodating a thermal sensor 65, is attached to the ceramic transitionpiece 63. The reglets 12, 13 consist of piezo ceramic material andmeasure, e.g., 70 mm by 1.6 mm×0.65 mm. Provided on their broadsideswith silver electrodes, the reglets 12, 13 have attached to them, in thearea of their bottom ends, the quartz tubelets 10, 11, their outsidediameter amounting to 1 mm and their inside diameter to 0.7 mm. Thequartz tubelets 10, 11 extend up to the outside wall of the quartzhollow cylinder 7 which serves to hold the specimen to be measured andhas an inside diameter of, e.g., 6 mm. FIG. 5 clearly depicts the wireloop 33 which is applied on the outside wall of the quartz hollowcylinder 7 and the twisted leads 68 of which are run along the quartztubelet 10. Both the wire loop 33 and the twisted lead 68 consist ofenameled gold wire measuring 0.05 mm in diameter.

The silver electrodes applied on the broadsides of the piezoelectricreglets 12, 13 and serving to tap the charges generated in the bendingdue to the piezoelectric effect are connected with the lines 14, 15 anda common line 69 for the internal silver electrodes, which carry thesame potential. The tongue 8 illustrated in FIGS. 5 and 6, representingthe oscillating part of the magnetic sensor, is about 10 cm long.

We claim:
 1. Magnetometer with an inhomogeneous magnetic field whichperiodically changes in intensity and which acts upon a specimen to bemeasured, the specimen attached thereto, the magnetometer comprising:anoscillatable tongue having a piezo element for sensing the tongueoscillations and for producing piezo signals in response thereto; anevaluation circuit connected to said piezo element, said evaluationcircuit includes a measuring amplifier for measuring the amplitudes ofsaid piezo signals; a low frequency generator connected to saidmeasuring amplifier; a power amplifier connected to said low frequencygenerator; a plurality of coils disposed around said oscillatabletongue, said coils connected to said power amplifier, and adapted togenerate a periodically changing magnetic field; a compensating currentloop for surrounding the specimen connected to said oscillatable tonguecharacterized in that the low frequency generator is activated by anautomatic frequency control circuit for adjusting the frequency of themagnetic field to the resonance frequency of the tongue; means forcomparing the low frequency generator output to said piezo signals andfor generating a direct current signal; and means for measuring themagnitude of the direct current through the compensating current loop.2. Magnetometer according to claim 1, characterized in that themeasuring amplifier is a phase-sensitive amplifier having a first outputsignal for energizing the compensating current loop.
 3. Magnetometeraccording to claim 2, characterized in that the phase-sensitiveamplifier has a second output signal indicative of the phase shift ofthe low frequency generator output relative to the piezo signals, saidsecond output signal being supplied to the automatic frequency controlcircuit.
 4. Magnetometer according to claim 3, including first andsecond switches which are respectively connected to two output lines ofsaid phase sensitive amplifier.
 5. Magnetometer according to claim 1including a second piezo element and wherein said two piezo elements arecoupled to said tongue, said piezo elements connected in opposite phaseto a charge/voltage converter, the piezo elements generating outputsignals of opposite phase as the tongue oscillates, said piezo outputsignals connected to a subtracting stage having an output signal whichforms an input signal for said measuring amplifier.
 6. Magnetometeraccording to claim 1 characterized in that the compensating current loopis actuated by a current measuring device and a current actuator whoseinput voltage is generated by a proportional/integral member which isconnected with the output of the measuring amplifier.
 7. Magnetometeraccording to claim 6, characterized in that the current actuatorincludes a second input for connecting to a direct voltage and forgeneration of an additional moment or for compensation of the force onsaid tongue.
 8. Magnetometer according to claim 1 characterized in thatthe free end of the tongue includes a hollow cylinder for receiving aspecimen to be measured, the circumference of said hollow cylindersurrounded by a compensating current loop made from electricallyconductive material and having at least one winding turn. 9.Magnetometer according to claim 8, characterized in that the hollowcylinder is connected, through two quartz tubelets which extendgenerally parallel to each other, a plurality of strips of piezo ceramicmaterial which extend generally parallel to each other and which areattached to the front end of a holding tube.
 10. Magnetometer accordingto claim 9, characterized in that a thermal sensor tube extends parallelto the strips and the quartz tubelets, said thermal sensor tubeincluding a thermal sensor which is disposed adjacent to the hollowcylinder.
 11. Magnetometer according to claim 8 characterized in thatthe tongue is located in a tail part of a cryostat.
 12. Magnetometeraccording to claim 11, characterized in that the tongue is arrangedinside a cryostat internal tube which includes two radially opposedcooling troughs.
 13. Magnetometer according to claim 12, characterizedin that a thermal shielding tube is arranged between the cryostatinternal tube and a cryostat external tube.
 14. Magnetometer accordingto claim 13, characterized in that the thermal shielding tube is athin-walled electrically conductive tube on which a plurality ofinsulated copper wires are helically wound.
 15. Magnetometer accordingto claim 14, characterized in that the ends of he copper wires, whichare adjacent the inside wall of the electrically conductive tube, arebent over at 90°, are clamped between two disks or rings, and areelectrically insulated from one another.